Image sensor with reduced spectral and optical crosstalk and method for making the image sensor

ABSTRACT

An integrated image sensor may include adjacent pixels, with each pixel including an active semiconductor region including a photodiode, an antireflection layer above the photodiode, a dielectric region above the antireflection layer and an optical filter to pass incident luminous radiation having a given wavelength. The antireflection layer may include an array of pads mutually separated by a dielectric material of the dielectric region. The array may be configured to allow simultaneous transmission of the incident luminous radiation and a diffraction of the incident luminous radiation producing diffracted radiations which have wavelengths below that of the incident radiation, and are attenuated with respect to the incident radiation.

TECHNICAL FIELD

Embodiments of the invention relate to optical image sensors, especiallyCMOS image sensors with front side or rear side illumination and, moreparticularly, to the reduction of crosstalk between adjacent pixels insuch sensors.

BACKGROUND

Referring to FIG. 1, an image sensor according to the prior art isshown. FIG. 1 provides a schematic sectional view of a first pixel 1,adjacent to a second pixel 2 of a CMOS image sensor of a type with frontside illumination (FSI). The two pixels have analogous structures. Thetwo pixels may, for example, form part of a Bayer pattern, which is wellknown to the skilled artisan.

The first pixel 1 is produced on a semiconductor substrate 10, withinwhich is a photoreceptor, e.g., a photodiode. The substrate 10 issurmounted by an interconnection part or portion 11, commonly referredto as the BEOL (Back End Of Line). An antireflection layer 4 may beproduced between the photodiode and the interconnection portion toensure good transmission of the light rays.

The interconnection portion 11 includes various metallization levels M1,M2 and vias shrouded in one or more dielectric materials. Color filters12 and 22 are situated on the interconnection portion 11, facing thephotodiodes. It should be noted that the metallization levels M1 and M2are produced in such a way that they are not situated in the regionbetween the optical filter and the photodiode. Only a dielectric regioncovers the antireflection layer 4. The pixel is surmounted by acollimation lens 13, making it possible to optimize the collection ofthe light rays at the level of the photodiode.

The color filter 12 of the first pixel 1 may, for example, be configuredto allow the wavelength corresponding to the color red to pass, and thefilter 22 of the second pixel 2 may be configured to allow thewavelength corresponding to the color green to pass.

During operation, the photons absorbed by the photodiode cause thegeneration of charge carriers, which create an electric current in thephotodiode. The mutual proximity of the pixels may give rise to aparticularly significant crosstalk phenomena in image sensors havingreduced or relatively small dimensions. Crosstalk arises when an opticalsignal 3 arriving, for example, at the first pixel 1 is not totallycollected by the corresponding photodiode 10, thus degrading theperformance of the sensor, especially the color rendition.

An optical crosstalk is when the photons passing through a filter reachthe photodiode of an adjacent pixel. For example, the optical signal 32,after having been filtered by the optical filter 22 of the second pixel2 (optical signal 31), reaches the photodiode 10 of the first pixel 1instead of reaching the photodiode 20 of the second pixel 2.

A spectral crosstalk is when the optical filter is not selective enoughand allows through wavelengths for which it is not configured. Forexample, a luminous signal, after passing within the green filter maycomprise wavelengths below or above that of green.

The crosstalk may also be of electrical origin when the electronsgenerated by the photodiode of a pixel disperse in an adjacent pixel.This crosstalk of electrical origin is not the subject of the presentpatent application.

In FIG. 2 the curves of quantum efficiency of a set of pixels forming aBayer pattern are illustrated. The three curves B, G, R correspondrespectively to the quantum efficiency of the blue, green and red pixelsof the sensor. The part G1 of the curve G shows that the green pixeldetects a significant share of signals whose wavelength corresponds tothe color blue. In an analogous manner, the part R1 of the curve R showsthat the red pixel detects a significant share of signals, including thewavelength corresponding to the color green. These spurious detectionsattest to the crosstalk phenomena explained above.

SUMMARY

According to one embodiment, an image sensor is provided which helps toreduce the phenomena of optical and spectral crosstalk, and therefore toobtain curves of quantum efficiency B, G, R having steeper slopes.

According to one aspect, an integrated image sensor is providedincluding adjacent pixels, with each pixel including a semiconductoractive region containing a photodiode, an antireflection layer above thephotodiode, a dielectric region above the antireflection layer, and anoptical filter to allow an incident luminous radiation having a givenwavelength to pass.

According to a general characteristic of this aspect, the antireflectionlayer may include an array of pads mutually separated by a dielectricmaterial of the dielectric region. The array may be configured to allowat the same time a transmission, optimized to the extent possible, ofthe incident luminous radiation and a diffraction of the incidentluminous radiation producing diffracted radiations which havewavelengths below that of the incident radiation and are attenuated withrespect to the incident radiation.

Thus, the phenomena of optical and spectral crosstalk are reduced byproposing an antireflection layer providing the dual function ofantireflection to ensure desired transmission of the incident radiationand of a high-pass filter to eliminate, to the extent possible, thewavelengths below the incident wavelength.

According to one embodiment, the pad array is periodic, and its periodis less than the ratio between the given wavelength and the sum of therefractive index of the material of the active region and of the productof the refractive index of the dielectric material times the sine of theangle of incidence of the incident radiation.

According to another embodiment, the height and the diameter of the padsmay be chosen in such a way that the refractive index of theantireflection layer is the closest to the square root of the product ofthe refractive index of the material of the active region times therefractive index of the dielectric material. Advantageously, the padsmay be circular. As such, the antireflection layer exhibits a symmetricstructure which confers on it isotropic behavior.

The pads may comprise silicon or polysilicon, although other materialsmay also be used.

Each pixel may further include a microlens above the correspondingfilter. This makes it possible to converge the light rays towards thephotodiode and therefore to optimize the collection of the signal on theantireflection layer.

According to one embodiment, the area of the antireflection layeroccupied by pads can be adapted to the wavelength to be filtered. Forexample, for a given wavelength close to 450 nanometers (i.e., close tothe color blue), 30% of the area of the antireflection layer of thecorresponding pixel may comprise pads; for a given wavelength close to540 nanometers (i.e., close to the color green), 49% of the area of theantireflection layer of the corresponding pixel may comprise pads; andfor a given wavelength close to 610 nanometers (i.e., close to the colorred), 56% of the area of the antireflection layer may comprise pads.These values in % are merely examples, and may be adapted by the skilledartisan for different embodiments as a function of the material of thepads, the thickness of the pads, the period of the pads, and theincident wavelength.

The image sensor may be of type with front side illumination, or withrear side illumination.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages and characteristics will become apparent upon examiningthe detailed description of non-limiting embodiments and the appendeddrawings, in which:

FIG. 1 is a schematic block diagram of an image sensor in accordancewith the prior art.

FIG. 2 is a graph illustrating curves of quantum efficiency of a set ofpixels such as in the image sensor of FIG. 1 forming a Bayer pattern inaccordance with the prior art.

FIG. 3 is a schematic cross-sectional diagram of a pixel of a CMOS imagesensor with front side illumination in accordance with an exampleembodiment.

FIG. 4 is perspective view of the antireflection layer of the pixel ofFIG. 3 in greater detail.

FIG. 5 is a schematic cross-sectional view of the antireflection layerof the pixel of FIG. 3 in greater detail.

FIG. 6 is a graph illustrating the transmittance of the antireflectionlayer for an incident luminous signal in the device of FIG. 3.

DETAILED DESCRIPTION

FIG. 3 illustrates a schematic section through a pixel 5 of a CMOS imagesensor with front side illumination, which illustratively includes anantireflection layer. The pixel is laterally insulated from theneighboring pixels by a deep trench isolation (DTI) 21. The pixelincludes, above a substrate 6, an active layer 7 (e.g., P typesconductivity) including a buried zone 8 (e.g., N type conductivity) inproximity to the upper side of the layer 7, e.g., forming a photodiode.

The upper side of the layer 7 supports, for example, a transfertransistor 9 and an antireflection layer 13 including pads 24 situatedabove the photodiode 8. The transistor 9 and layer 13 are wrapped in orcovered by an insulating layer 14, e.g., PreMetal Dielectric (PMD). Thisinsulating layer includes a first dielectric material, e.g., silicondioxide.

The PMD layer 14 is surmounted or covered by an interconnection part orportion 15, commonly referred to as the Back End Of Line (BEOL). Thepart 15 includes various metallization levels. In the present example,there are three levels M1, M2 and M3, each comprising electricallyconducting tracks 16 as well as vias 18 shrouded in a dielectric regioncomprising a second dielectric material 17, commonly referred to asInterMetal Dielectric (IMD).

The pixel 5 also illustratively includes a color filter 19 situatedabove the last metallization level M3 facing the photodiode 8. Thisfilter is configured to allow through only certain wavelengths of aluminous signal. For example, here the wavelengths close to 540nanometers corresponding to the color green are allowed to pass.

A collimation lens 20 is advantageously disposed above the opticalfilter 19. The lens directs the incident rays towards the photodiode 8to the maximum extent possible.

Referring to FIGS. 4 and 5, the antireflection layer 13 is illustratedin greater detail. The layer 13 is produced or formed on the substrate 7and includes the array of pads 24, e.g., circular polysilicon pads. Theshape of the pads 24 is not limited to a circular shape, rather the padsmay be various different shapes. However, a circular shape makes itpossible to have a symmetric structure of the antireflection layer, andthus isotropic behavior. The pads 24 are shrouded in the silicon oxideof the insulating layer 14, which is represented as transparent in FIG.3 for simplification.

The pads 24 may, for example, be formed after the deposition of theinsulating layer 14 by etching the layer 14 at the location of the pads,then by filling the orifices thus obtained with polysilicon according tothe desired pad thickness. The orifices above the polysilicon pads maythen be plugged with silicon oxide. Finally, a step ofchemical-mechanical planarization (CMP) of the structure is applied.

The refractive index n of the antireflection layer disposed between thesubstrate 7 (which has a refractive index n_(s)) and the layer 14 (whichhas a refractive index n_(i)) will ideally satisfy the formulan=√{square root over (n _(i) *n _(s))}For silicon in the wavelengths close to green, for example,theoretically n_(s) is equal to about 4 and the theoretical index n_(i)of the dielectric PMD is equal to about 1.5.

Furthermore, for such an array structure, diffraction orders appear fora light ray of a given wavelength λ and angle of incidence θ when theperiod b of the array satisfies the equation:

$b < \frac{\lambda}{\left( {n_{s} + {n_{i}*{\sin(\theta)}}} \right)}$For example, for a pixel with a green color filter, the pads 24 areorganized here as a regular array of period b equal to 100 nanometers.Their height h is 50 nanometers, and their diameter d is 70 nanometers.

One advantage of an array structure is that, by adapting the ratiobetween the area occupied by the pads with respect to the total area, itis possible to vary the ratio between the quantity of polysilicon of thepads and the quantity of silicon oxide of the PMD layer 14 present inthe antireflection layer. It is therefore possible to adapt therefractive index of the antireflection layer by configuring the diameterd of the pads to obtain the desired refractive index n, which will bebetween 1.5 and 4.

By way of example, 49% of the area of the antireflection layer occupiedby pads 24 leads to a configuration best adapted for the transmission ofsignals of wavelength close to 540 nanometers (green). In this example,and with the values of b, h, and d mentioned above, the refractive indexn of the antireflection layer is in the vicinity of 2.4.

When an incident light ray 25 arrives at the antireflection layer 13, itis transmitted within the substrate 7 in the form of a transmitted raym₀, reflected by the antireflection layer in the form of a reflected raym_(r) and diffracted as several diffracted rays m₁, m₂, m₃, m₄ . . . ofdifferent wavelengths, each corresponding to a different mode ofdiffraction and order. The incident ray 25 may, for example, havepreviously passed through the optical filter 19 and because the filteris not perfect, includes wavelengths remote from 540 nanometers.

The transmitted light ray m₀, corresponding to the wavelength for whichthe filter is configured (here the wavelengths close to 540 nanometersand corresponding to the color green), deviates very little, if at all.The diffracted rays of higher orders, corresponding to the signals ofwavelengths below the desired wavelength, undergo a diffractionproportional to their diffraction order. Thus, the second-order ray m₂has a lower transmittance than the ray m₁ of order one, and so on and soforth.

FIG. 6 schematically represents the transmittance of the antireflectionlayer 14 for the incident luminous signal. Here, the best transmittancecorresponds to the transmitted signal m₀ of wavelength 540 nanometers.It may be observed that the luminous signals m₁, m₂, m₃, m₄,respectively of orders 1, 2, 3, 4, are very attenuated with respect tothe main order, and appear for wavelengths below 540 nm. Thus, theantireflection layer 13 behaves as a high-pass filter, attenuating oreliminating the signals of wavelengths below the desired wavelength,here 540 nanometers.

The antireflection layer may also be adapted to the color blue, in whichcase 30% of the area of the layer is occupied by pads 50 nanometers inheight and 55 nanometers in diameter, and the period of the array is 100nanometers. In this example, the refractive index of the antireflectionlayer is in the vicinity of 3.2.

In another example embodiment, the antireflection layer may also beadapted to the color red, in which case 56% of the area of the layer isoccupied by pads 50 nanometers in height and 225 nanometers in diameter,and the period of the array is 300 nanometers. In this example, therefractive index of the antireflection layer is in the vicinity of 2.7.

These various values are merely indicative and should be adapted by theperson skilled in the art as a function of the transmission of the layerand of the cutoff wavelength which are desired in different embodiments.Nonetheless, using this approach it is possible to reduce the spectraland optical crosstalk by attenuating the humps G1 and R1 of FIG. 2.

Although the pixel 5 exhibited in these examples is of a type with frontside illumination, the antireflection layer 13 may also be integratedinto pixels of a type with rear side illumination. In this case, thepads are obtained by depositing a layer of polysilicon, for example,according to the desired thickness h, then by the etching the layer todefine the geometry of the pads. Further, the dielectric material of thePMD insulating layer is deposited. A step of chemical-mechanicalplanarization (CMP) of the structure may then be performed.

That which is claimed is:
 1. A method for making an image sensorcomprising: forming a plurality of pixels to one another and eachcomprising an active semiconductor active region; and defining aphotodiode, an antireflection layer above the photodiode, a dielectricregion above the antireflection layer, and an optical filter to pass anincident luminous radiation having a given wavelength associatedtherewith, the antireflection layer comprising an array of padsseparated by adjacent portions of the dielectric region, the array ofpads positioned to allow simultaneous transmission of the incidentluminous radiation and a diffracted radiation from the incident luminousradiation, the diffracted radiation having wavelengths below that of thegiven wavelength associated with the incident radiation and also beingattenuated with respect to the incident radiation.
 2. The method ofclaim 1 wherein the array of pads is periodic with a period b such that:${b < \frac{\lambda}{\left( {n_{s} + {n_{i}*{\sin(\theta)}}} \right)}},$where λ is the given wavelength, n_(s) is a refractive index of theactive semiconductor region, n_(i) is a refractive index of thedielectric region, and θ is an angle of incidence of the incidentradiation.
 3. The method of claim 1 wherein the pads comprise circularsilicon pads.
 4. The method of claim 1 wherein for a given wavelength ina range of 450 to 610 nanometers, an area of the antireflection layer ofthe associated pixel comprises pads is in a range of 30-56%.
 5. An imagesensor comprising: a plurality of pixels adjacent to one another andeach comprising an active semiconductor region defining a photodiode, anantireflection layer above the photodiode, a dielectric region above theantireflection layer, and an optical filter to pass an incident luminousradiation having a given wavelength associated therewith, saidantireflection layer comprising an array of pads separated by adjacentportions of said dielectric region, the array of pads positioned toallow simultaneous transmission of the incident luminous radiation and adiffracted radiation from the incident luminous radiation, thediffracted radiation having wavelengths below that of the givenwavelength associated with the incident radiation and also beingattenuated with respect to the incident radiation.
 6. The image sensorof claim 5 wherein said pads are circular.
 7. The image sensor of claim5 wherein said pads comprise silicon.
 8. The image sensor of claim 5wherein each pixel further comprises a microlens above said opticalfilter.
 9. The image sensor of claim 5 wherein for a given wavelength of450 nanometers, 30% of an area of said antireflection layer of theassociated pixel comprises pads.
 10. The image sensor of claim 5 whereinfor a given wavelength of 540 nanometers, 49% of the area of saidantireflection layer of the associated pixel comprises pads.
 11. Theimage sensor of claim 5 wherein for a given wavelength of 610nanometers, 56% of the area of said antireflection layer of theassociated pixel comprises pads.
 12. The image sensor of claim 5 whereinfor a given wavelength in a range of 450 to 610 nanometers, an area ofsaid antireflection layer of the associated pixel comprising pads is ina range of 30-56%.
 13. The image sensor of claim 5 wherein the imagesensor comprises a front side illumination sensor.
 14. The image sensorof claim 5 wherein the image sensor comprises a rear side illuminationsensor.
 15. The image sensor of claim 5 wherein the array of pads isperiodic with a period b such that:${b < \frac{\lambda}{\left( {n_{s} + {n_{i}*{\sin(\theta)}}} \right)}},$where λ is the given wavelength, n_(s) is a refractive index of saidactive semiconductor region, n_(i) is a refractive index of saiddielectric region, and θ is an angle of incidence of the incidentradiation.
 16. The image sensor of claim 15 wherein a height anddiameter of the pads define a refractive index n of the antireflectionlayer such that:n=√{square root over (n _(i) *n _(s))}.
 17. An image sensor comprising:a plurality of pixels adjacent to one another and each comprising anactive semiconductor region defining a photodiode, an antireflectionlayer above the photodiode, a dielectric region above the antireflectionlayer, and an optical filter to pass an incident luminous radiationhaving a given wavelength associated therewith, said antireflectionlayer comprising an array of circular silicon pads separated by adjacentportions of said dielectric region, the array of pads positioned toallow simultaneous transmission of the incident luminous radiation and adiffracted radiation from the incident luminous radiation, thediffracted radiation having wavelengths below that of the givenwavelength associated with the incident radiation and also beingattenuated with respect to the incident radiation.
 18. The image sensorof claim 17 wherein the array of pads is periodic with a period b suchthat:${b < \frac{\lambda}{\left( {n_{s} + {n_{i}*{\sin(\theta)}}} \right)}},$where λ is the given wavelength, n_(s) is a refractive index of saidactive semiconductor region, n_(i) is a refractive index of saiddielectric region, and θ is an angle of incidence of the incidentradiation.
 19. The image sensor of claim 17 wherein each pixel furthercomprises a microlens above said optical filter.
 20. The image sensor ofclaim 17 wherein for a given wavelength in a range of 450 to 610nanometers, an area of said antireflection layer of the associated pixelcomprises pads is in a range of 30-56%.